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Cascading Drain Calculation


i . How to apply cascading drain 

ii. How to determine number of cascading steps & length

iii. How to calculate IL for start node and end node

How to apply cascading drain #

Go to Options > Project Parameters > Drainage > Design > Cascading Drain


  1. User can set the ‘Maximum Ground Level Gradient’ and the ‘Cascading Drop’
  2. By default, the Maximum Ground Level Gradient is set to 1 to 50 while Cascading Drop is set to 150 mm. 


  • Cascading drain can be assign by Auto, Always Apply or Don’t Apply
    1. Auto – software will compare the gradient of GL between start node and end node. When the gradient exceeds maximum gradient allowed, then software will automatically apply cascading drain.
    2. Always Apply – cascading drain will ‘always’ be applied despite the GL gradient between start node and end node
    3. Don’t Apply – cascading drain will not be applied


User can also used the constraint parameter to apply cascading drain. This feature is very convenient for user to apply cascading drain to a specific drain.

Parameters such as the ‘Maximum Gradient Ground Level’ and ‘Cascading Drop’ can have different value from the value set in the general parameter.

Go to Drain > Spread Input > Constraint Parameter > Change from ‘Follow Parameter’ to ‘Customized’ > open Drain Constraint Dialog

Determine number of cascading steps and cascading length #

For cascading drain calculation; you may follow formula given below;

  1. Step number = (Start IL – endMaxIL – Distance*Gradient) / Cascading drop
  2. Step Length = Distance / Step Number
  3. Final IL = Start IL – [step number * [step length*(gradient + cascading drop)]


i. Distance is the drain length

ii. Gradient is the drain gradient

  • software will use the gentler gradient from the parameter or a fixed gradient set by user 
  • if the gradient is not admissible with the existing terrain and other constraints hence software will propose most gentle gradient that will satisfies all the constraints 
  • cascading drain gradient is the same as drain gradient selection, the selected gradient will be the most gentle provided all the constraints like velocity, drain capacity and terrain conditions are satisfied
  • as in this example, the most gentle gradient is 1:100 

iii. EndMaxIL = end node GL – design drain depth

How to calculate IL for start node and end node #

NODE 1516
GL (m)9.006.00
IL (m) 7.90EndMaxIL = 5.55

1. Step number

= [(Start IL – endMaxIL – (Distance*Gradient)] / Cascading drop

[7.9m – 5.55m – (40.85*1/100) / 0.3m]

6.47 steps ; take 7 steps

2. Step length

Distance / Step Number

40.85m / 7

= 5.83m

3. Final IL at NODE 16

Step No. IL (m)Drop (m) Step Length (m) Fall 1:100 (m) 
Start IL7.900.35.830.0583
End IL 75.380.3

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